Mare Barrow is born and raised in the Kingdom of Norta, which is characterized by a sharp class divide: the Silvers, who have silver blood, live lives of glamor and riches. Reds, who have red blood, live in poverty in villages like the Stilts. Mare, a Red who has grown up in the Stilts, must pick pockets to support her family. She brings stolen goods home every day to her mother, who disapproves of her daughter’s quasi-profession, her younger sister, Gisa, who has an actual job as an apprentice to a seamstress, and her father, who uses a wheelchair and an iron lung because he came home injured from war. Mare’s three older brothers, Bree, Tramy, and Shade, are absent because they, too, have been conscripted to act as faceless bodies in the ongoing war. The war has been raging on the border of Norta for over a hundred years, and Reds have no stake in it—except insofar as they provide the Silvers with the human fuel to keep their war going. Any Red who is not apprenticed or employed by the age of eighteen (which is most Reds, because jobs are few and far between, and the school system does not teach many skills that could win an apprenticeship) is sent to the war front. Most of them either do not come back, or come back severely injured. The Silvers have the resources to treat many of these injuries, but they choose not to invest in doing so, because there is a steady supply of fresh Reds from which to draw.
The Silvers are able to use the Reds in this way because the Silvers’ special gifts to manipulate fire, metal, earth, water, and more has given them enough of an edge over the gift-less Reds to force them into poverty. The Silvers stage Feats every month in giant arenas in Red villages, at which Silver champions fight one another. The spectacle is entertaining, but Mare knows that it doubles as a reminder to the Reds that they are powerless in the face of this fantastically gifted ruling class. Still, when Mare’s best friend, Kilorn, finds out that he is about to be conscripted, she decides that she will do everything she can to fight back. She goes to Will Whistle, the man who usually buys her stolen goods to sell on the black market, and asks if he can smuggle her and Kilorn out of the Stilts so that they might go on the run from the Silvers. Will refuses at first, but then introduces Mare to Farley, a woman who claims to be a first-rate smuggler. Farley accepts the task of smuggling Mare and Kilorn out of town, but only if Mare pays her two thousand crowns in the next two days. This sum is a veritable fortune to Mare, but she accepts Farley’s offer, resolving to steal the money.
Mare gets Gisa, who works in the Silver metropolis where the king lives during the summer, to help her sneak into the city. While Mare is attempting to pick pockets in a bar there, a broadcast comes on the television. Mare sees Farley claiming credit for a terrorist attack on the Silvers, on behalf of a group called the Scarlet Guard. The Guard is allegedly working to advance Red rights. Chaos breaks out in the bar and all over the city as Reds try to flee and Silvers capture them, torturing them for information they do not have on the Scarlet Guard. Mare finds Gisa and has almost managed to make it out of the city with her when Gisa decides to pick a pocket. Less adept than Mare, she is caught. As punishment, a Silver security officer smashes Gisa’s sewing hand.
That night, feeling that she has betrayed her family by getting Gisa into this mess, Mare sneaks off to some bars on the edges of the Stilts. She does not go to drink, but rather to steal from the patrons. The last person whose pocket she picks catches her. Instead of getting angry, he gives her a silver coin, walks her home, and gives her a second silver coin. He introduces himself as Cal, and says that he works at the palace. Mare does not think she will ever hear anything from him again, but in the morning, security officers and a royal servant show up at the Barrow house to escort Mare to the palace. At first, she is terrified that she is about to be tortured for information on Farley. However, she soon finds that Cal has arranged for her to have a serving job at the palace.
Mare’s first task as a royal servant is to serve the Silver crowd at an event called Queenstrial, at which the eligible young Silver noblewomen compete for the two princes’ hands in marriage. Mare runs around a stadium similar to, but far more ornate than, that in which she has seen the Feats every month in the Stilts. When the royal family enters, Mare feels shocked and betrayed to find that the crown prince is Cal. As she reels from this discovery, she observes the young women competing in the depressed arena below, separated from the crowd by a vast electric shield. It strikes her that the Silvers are more powerful than she ever thought. The last contestant, Evangeline Samos, seems favored to win, although her power of manipulating metal does not impress Mare as much as some of the others’ powers. Evangeline then makes the entire arena begin to collapse in on itself. Mare falls over the edge and has resigned herself to dying upon collision with the electric shield. When she collides with it, however, it is not Mare but rather the shield that suffers the most damage. She seems to burn through it. Before all the Silvers’ eyes, she falls through the shield to the arena below. Evangeline, terrified, goes on the offensive. Mare deflects her shards of metal with some sort of electric shield of her own. King Tiberias yells for sentinels to seize Mare, and she runs. However, Cal catches her by using his own gift: manipulating flames.
Mare wakes to find that Queen Elara has invaded her head using her talents as a “whisper” who can read and control minds. Elara and the king seem threatened by Mare. They tell her that from now on, they will conceal her identity as a Red by peddling the story that she is a Silver whose parents died in the war during her infancy; she was raised by Reds who concealed her identity from her until now. She will marry the second-born prince, Maven, and live the rest of her life as a Silver.
Mare does not feel like she is living a fairy tale, but rather like she is a prisoner. As she learns the ropes of life at the royal court, she constantly looks over her shoulder, sure that she is going to be betrayed by someone close to her. Despite her mistrust of them, she feels alternately drawn to both the princes. She eventually decides to trust Maven, who claims to work with the Scarlet Guard and desire equality for Reds and Silvers. Mare herself joins the Scarlet Guard and becomes party to a terrorist attack at the palace. In the aftermath of the attack, however, she feels unsure that she should have trusted Maven. Cal, by contrast, is willing to maintain the unjust status quo, but his objections to murder seem more in line with Mare’s views. Throughout the novel, as Mare attempts to learn who she is and what her values are, she remains unsure of who her actual friends are, apart from the direct family members she is no longer allowed to see.
Finally, Mare decides that Maven’s radical views are the only pathway to true justice. She chooses him over Cal, both politically and romantically. Mare makes one final appeal to Cal to convince his father to be a more just ruler, but Cal declines. Just when Mare feels sure that she has chosen the right brother, Maven reveals that he has been in league with his mother, Queen Elara, the whole time. They have been planning a coup, not to instate a more just regime that treats Reds as equal citizens, but rather to put Maven on the throne for personal gain. Queen Elara controls Cal’s mind, forcing him to kill the king, his father. She then immediately yells out in shock and grief so that sentinels rush in and believe that Cal and Mare are the traitors.
The novel ends with Cal and Mare on a train with members from the Scarlet Guard, who have snuck them out of the capital. Mare finds out that the Silvers have kept from her an additional secret about her identity: she is not the only Red who displays Silver talents. Silvers have been keeping on lockdown that there is an entire subset of citizens with red blood who also have the genetic markers of these magical gifts. Mare finds out that her brother Shade, who she thought was executed for involvement with the Scarlet Guard, is in fact alive and one of them. Mare decides that she should not trust any one person, but that the work the Scarlet Guard is doing to organize a Red resistance underground is the work she wants to do as well. She could never be romantically involved with Cal, she decides, because they have betrayed one another too many times. However, she knows that they share a goal moving forward: both of them have a vendetta against Maven. If they ever get close to him, they will kill him.